1. Field of the Invention
This invention relates to modulators and more specifically to pre-distorted 12/4-QAM modulators.
2. Background and Material Information
Although quadrature amplitude modulation (QAM) techniques are recognized to be bandwidth-efficient, these techniques are usually implemented with linear amplifiers to prevent introducing nonlinear distortion into the constellation. For space-to-ground applications, this is very inefficient. While nonlinear amplifiers are more efficient, they distort the QAM constellation. Generally, when bandwidth efficiencies of four bits per symbol is desired, 16 QAM is used because they are relatively simple to synthesize. However, 16 QAM has some undesirable characteristics. Furthermore, 16 QAM has multiple amplitude levels. If a signal is amplified with a saturating amplifier (e.g., on a space to earth downlink), when using 16-QAM, it may be required to back off or reduce the amplification so that the constellation is not distorted.
Further, conventional predistortion techniques, such as those used with 16-QAM, independently predistort the I and Q components of each constellation point. Hence, for a QAM modulation approach such as 16-QAM, 32 independent controls may be required to control the predistortion of the I and Q components. This may translate into additional hardware that is needed in the modulator. In some communication systems, such as satellite communication systems, additional hardware increases weight and power consumption, both of which may have limitations. Further, additional hardware increases the overall cost of the system.
Moreover, in some implementations of 16-QAM modulators with predistortion, for every symbol, a table is used to map the four data bits to a transmitted amplitude and phase. This mapping requires additional weight and power consumption which may be limited and increase the overall cost of the system.
The present invention is directed to a predistorted 12/4-Quadrature Amplitude Modulation (QAM) modulator, which may include a mapping device that maps input bits to a plurality of modulator control bits. The mapping may be determined by selected desired points on a 12/4-QAM constellation. At least one phase shift device may receive an input signal and at least one of the plurality of NRZ symbols. At least two quaternary phase shift keying (QPSK) devices may receive phase shifted signals from the at least one phase shift device. Each at least two QPSK devices may receive at least one of the plurality of NRZ symbols. An attenuator may attenuate a first QPSK signal outputted from a first QPSK device of the at least two QPSK devices. A summer may add the attenuated first QPSK signal with a second QPSK signal. The second QPSK signal may be outputted from a second QPSK device of the at least two QPSK devices. The summer outputs a predistorted 12/4-QAM of the modulator input bits. The plurality of NRZ symbols may control the at least one phase shift device and at least two QPSK devices to achieve the desired points on the 12/4-QAM constellation.
The attenuator may be a variable attenuator. At least one of the plurality of NRZ symbols may control at least one phase shift device to shift the input signal 30, 60, 120, or 150 degrees. At least one of the plurality of NRZ symbols may control the first QPSK device to shift the received phase shifted signals 45 degrees, 135 degrees, 225 degrees, or 315 degrees. At least one of the plurality of NRZ symbols may control the second QPSK device to shift the received phase shifted signals 45 degrees, 135 degrees, 225 degrees, or 315 degrees.
A splitter may split the phase shifted signals from at least one phase shift device into two phase shifted signals. The first QPSK device may receive one of the two phase shifted signals. The second QPSK device may receive a second of the two phase shifted signals. The first QPSK device and/or the second QPSK device may split the phase shifted signals into two phase shifted signals. An upper signal may be generated from one of the two phase shifted signals being shifted 0 degrees or 180 degrees. A lower signal may be generated from a second of the two phase shifted signals being shifted +90 degrees or xe2x88x9290 degrees. The first QPSK signal may be generated from adding the upper signal and the lower signal. The second QPSK signal may be generated from adding the upper signal and the lower signal.
The attenuator may include: a first switch that may receive the first QPSK signal; a plurality of attenuation elements where each of the plurality of attenuation elements may be operatively connected to the first switch, and where each of the plurality of attenuation elements may produce a different attenuation; a second switch operatively connected to each of the plurality of attenuation elements; and a controller where the controller may control the first switch and the second switch to cause the first QPSK signal to pass through one of the plurality of attenuation elements that attenuates the first QPSK signal.
The attenuator may also include: a variable gain amplifier that receives the first QPSK signal; a D/A converter operatively connected to the variable gain amplifier; and a controller that may control the variable gain amplifier to attenuate the first QPSK signal. The controller may control the variable voltage amplifier using the D/A converter. At least one phase shift device and at least two QPSK devices operate at a predistorted 12/4 QAM modulator symbol rate. The desired points on the 12/4 QAM constellation may be chosen so the number of bits of each desired point where adjacent desired points differ is minimized.
According to the present invention, a method for predistorted 12/4-Quadrature Amplitude Modulation (QAM) may include: selecting desired points on a 12/4-QAM constellation; mapping an input signal to a plurality of nonreturn-to-zero (NRZ) symbols where the mapping may be determined by the selected desired points; phase shifting a continuous wave (CW) signal where the phase shifting may be controlled by at least one of the plurality of NRZ symbols; splitting the phase shifted CW signal into a first phase shifted CW signal and a second phase shifted CW signal; performing quaternary phase shift key (QPSK) modulation on the first phase shifted CW signal and the second phase shifted CW signal where the QPSK modulator may be controlled by at least two of the plurality of NRZ symbols, and the QPSK modulator may generate a first QPSK signal and a second QPSK signal; attenuating the first QPSK signal; and adding the attenuated QPSK signal to the second QPSK signal where the addition may generate a predistorted 12/4-QAM signal with the desired points.
The CW signal may be phase shifted 30, 60, 120, or 150 degrees. The QPSK modulator may shift the first phase shifted CW signal 45, 135, 225, or 315 degrees. The QPSK modulator may shift the second phase shifted CW signal 45, 135, 225, or 315 degrees.
The QPSK modulator may include: splitting the first phase shifted signal into an upper signal and a lower signal; shifting the upper signal 0 degrees or 180 degrees; shifting the lower signal +90 degrees or xe2x88x9290 degrees; and adding the shifted upper signal and the shifted lower signal. The QPSK modulator may include: splitting the second phase shifted signal into an upper signal and a lower signal; shifting the upper signal 0 degrees or 180 degrees; shifting the lower signal +90 degrees or xe2x88x9290 degrees; and adding the shifted upper signal and the shifted lower signal. The phases shifting and the QPSK may occur at a modulator symbol rate.
The present invention is also directed to a predistorted 12/4 Quadrature Amplitude Modulation (QAM) modulator that may include: a first phase shift module where the first phase shift module receives an input signal and a fifth NRZ symbol; a second phase shift module operatively connected to an output of the first phase shift module where the second phase shift module receives a sixth NRZ symbol; a first QPSK module operatively connected to an output of the second phase shift module where the first QPSK module receives a phase shifted signal from the second phase shift module, and the first QPSK module receives a first NRZ symbol and a second NRZ symbol; a second QPSK module operatively connected to the output of the second phase shift module where the first QPSK receives the phase shifted signal from the second phase shift module, and the second QPSK module receives a third NRZ symbol and a fourth NRZ symbol; a variable attenuator where the variable attenuator may be operatively connected to the second QPSK module; and a summer operatively connected to the first QPSK module and the variable attenuator where the summer produces a 12/4 QAM signal by adding a QPSK signal from the first QPSK module with an attenuated signal from the variable attenuator. An input signal may be mapped to the first NRZ symbol, second NRZ symbol, third NRZ symbol, fourth NRZ symbol, fifth NRZ symbol, and sixth NRZ symbol based on desired points on a 12/4 QAM constellation. The first NRZ symbol, second NRZ symbol, third NRZ symbol, fourth NRZ symbol, fifth NRZ symbol, and sixth NRZ symbol may control the 12/4 QAM modulator to produce the desired points on the 12/4 QAM constellation.
The present invention is further directed to a pre-distortion QAM modulator that may include: at least one phase shift module operating at a modulator symbol rate where the at least one phase shift module may receive an input signal; at least two quaternary phase shift keying modules operating at the modulator symbol rate where the at least two quaternary phase shift keying modules may be operatively connected to the at least one phase shift module, and each at least two quaternary phase shift keying modules may receive a different portion of a signal from the at least one phase shift module, and the at least one phase shift module and the at least two quaternary phase shift keying modules may receive at least one nonreturn-to-zero (NRZ) symbol input; a variable attenuator device operatively connected to one of the at least two quaternary phase shift keying modules; a summer operatively connected to the at least two quaternary phase shift keying modules where the summer may produce a modulator output signal by adding output signals from the at least two quaternary phase shift keying modules. The modulator output signal may be a pre-distorted QAM modulation for the at least one NRZ symbol that compensates for amplitude compression due to distortion from a nonlinear element between the modulator output signal and a demodulator.
The present invention is directed to a method for pre-distorted QAM modulation that includes: receiving a carrier signal and at least one input symbols; phase modulating the carrier signal at an input symbol rate; splitting the phase modulated carrier signal into parts; modulating each phase modulated carrier signal part at the input symbol rate using quaternary phase shift keying; attenuating one of the quaternary phase shift keying modulated phase modulated carrier signal parts; and summing the attenuated one of the quaternary phase shift keying modulated phase modulated carrier signal parts with each other quaternary phase shift keying modulated phase modulated carrier signal part. The summing may produce a pre-distorted QAM modulation for the at least one input symbol that compensates for amplitude compression due to demodulator nonlinear distortion.